1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system having the same.
2. Description of the Related Art
Example methods are known in which a diffractive optical element having a diffractive effect is provided at a part of a lens system to reduce chromatic aberration of the lens system (see SPIE Vol. 1354, International Lens Design Conference, 1990, Japanese Patent Laid-Open No. 4-213421 corresponding to U.S. Pat. No. 5,044,706, Japanese Patent Laid-Open No. 6-324262 corresponding to U.S. Pat. No. 5,790,321, and U.S. Pat. No. 5,044,706). This method with the diffractive optical element utilizes a physical phenomenon that chromatic aberrations appear in opposite directions on a refractive surface and a diffractive surface in an optical system when a light beam with a reference wavelength is applied.
Since the diffractive optical element may provide an effect similar to an effect of an aspherical lens by properly changing a cycle of the cyclic structure of the diffractive optical element, the effect may reduce aberrations other than the chromatic aberration.
In a lens system having such a diffractive optical element, when a light beam in a usable wavelength region is concentrated to diffractive light of a certain order (also referred to as predetermined order or designed order), the intensity of diffractive light of other orders becomes low. If the intensity is 0, it means that the diffractive light does not exist. However, unnecessarily diffracted light of the orders other than the designed order actually exists. However, in fact, if the unnecessarily diffracted light has a certain intensity, the light is emitted through the optical system in a path different from that of the light beam of the designed order, resulting in flare.
Because of this, to utilize the aberration reducing effect by way of the diffractive optical element, a diffractive efficiency of the diffractive light of the designed order is required to be reliably high over the usable wavelength region. Also, it is important to carefully consider about a spectral distribution of the diffractive efficiency of the designed order, and a behavior of the unnecessarily diffracted light of the orders other than the designed order.
Various suggestions are made for diffractive optical elements to improve the diffractive efficiency and reduce the unnecessarily diffracted light (see Japanese Patent Laid-Open No. 9-127322 corresponding to U.S. Pat. No. 6,157,488, Japanese Patent Laid-Open No. 2000-98118 corresponding to U.S. Pat. No. 6,560,019, and Japanese Patent Laid-Open No. 2002-107523 corresponding to U.S. Pat. No. 6,683,718). The diffractive optical elements disclosed in these publications have a plurality of layered diffraction gratings, and the material and height of each diffraction grating are properly determined. Accordingly, a high diffractive efficiency is provided for diffractive light of a desired order over a wide wavelength region (about 98% in a wavelength region ranging from 450 to 650 nm). The diffractive efficiency is expressed by a ratio of a light intensity of diffractive light of each order, to a light intensity of the total transmitted light beam.
In such a layered diffractive optical element, if a positional shift is found between the plurality of layered diffraction gratings, a diffractive efficiency of a light beam of an order different from the designed order increases. As a result, when the diffractive optical element is applied to the optical system, the image quality of an obtained image is seriously deteriorated. Therefore, to manufacture the layered diffractive optical element, it is important to highly accurately position the plurality of diffraction gratings to be layered. Example methods for manufacturing diffractive optical elements are known in which a plurality of diffraction gratings are highly accurately positioned and layered (see Japanese Patent Laid-Open No. 2001-141918 corresponding to US Patent Application Publication No. 2001/0026399, Japanese Patent Laid-Open No. 2002-62417 corresponding to U.S. Pat. No. 6,965,476, Japanese Patent Laid-Open No. 10-274705 with no corresponding US patent publication, Japanese Patent Laid-Open No. 2000-292668 with no corresponding US patent publication, and Japanese Patent Laid-Open No. 2000-114143 corresponding to U.S. Pat. No. 6,731,431).
To increase the diffractive efficiency, a distance (air gap) between the plurality of diffraction gratings to be layered should be small.
However, if the air gap becomes small, a grating surface of one diffraction grating may come into contact with a grating surface of the other diffraction grating, causing the grating surfaces to be damaged.
In a case where a thin substrate is used for making the plurality of diffraction gratings, if an external pressure is applied to the diffraction gratings, the substrate may be deformed. Accordingly, the grating surface of the one diffraction grating may come into contact with the grating surface of the other diffraction grating, causing the grating surfaces to be damaged.
To prevent this, a positioning member may be used as a support for preventing the deformation caused by the external pressure.
However, with this method, since the positioning member is generally used only at a small area, if a large external pressure is applied, the substrate may be markedly deformed, and the grating surfaces come into contact with each other, thereby causing the grating surfaces to be damaged.